The present invention relates to substance combinations as vapour phase corrosion inhibitors (corrosion inhibitors capable of evaporating or sublimating, vapour phase corrosion inhibitors VPCI, volatile corrosion inhibitors VCI) for protecting customary utility metals, such as iron, chromium, nickel, tin, zinc, aluminium, copper, magnesium and alloys thereof, against corrosion in humid climates.
Already for several decades, use has been made of corrosion inhibitors which tend to evaporate or sublimate even under normal conditions and thus can pass via the gas phase onto metal surfaces that are to be protected, for the temporary corrosion protection of metal objects within closed spaces, e.g. in packagings, switch cabinets or display cases. Protecting metal parts in this way against corrosion during storage and transport is known to be the clean alternative to temporary corrosion protection using oils, greases or waxes.
These corrosion inhibitors which preferably act via the vapour phase are usually selected depending on the type of metal to be protected and are used as a powder, packaged in bags made from a material that is permeable to the VCIs in vapour form (cf. for example: E. Vuorinen, E. Kalman, W. Focke, Introduction to vapour phase corrosion inhibitors in metal packaging, Surface Engng. 29 (2004) 281 pp.; U.S. Pat. No. 6,752,934 B2).
Modern packaging materials for corrosion protection contain the VCIs either as powder or tablets inside gas-permeable containers (e.g. paper bags, plastic capsules), coatings on paper, cardboard, foams or textile nonwovens, or incorporated directly within polymeric carrier materials. For instance, the patents U.S. Pat. Nos. 3,836,077, 3,967,926, 5,332,525, 5,393,457, 4,124,549, 4,290,912, 5,209,869, JP 2002253889 A, EP 0,639,657, EP 1,219,727, U.S. Pat. No. 6,752,934 B2, US 2009/0111901 A1 and DE-OS 102007059 726 A1 propose different variants for introducing the VCIs into capsules, coatings or gas-permeable plastic films so that in each case there is obtained a product from which the VCI components can continuously evaporate or sublimate.
The production of VCI-containing packaging materials by dissolving the VCI components in a suitable solvent and applying to a suitable carrier material is particularly obvious and has already been practised for a long time. Methods of this type using different active substances and solvents are described for example in JP 61,227,188, JP 62,063,686, JP 63,028,888, JP 63,183,182, JP 63,210,285, U.S. Pat. Nos. 3,887,481 and 5,958,115.
Finally, there is an increasing requirement to provide VCI-containing oils. In this case, the films of oil applied to metal surfaces are intended to protect against corrosion not only the metal substrate in question but rather also surface regions of the metals in question which could not be coated with a film of oil due to their geometry (e.g. holes, narrow notches, folded metal-sheet layers), since the VCI components emitted from the oil pass via the vapour phase to the oil-free surface regions within closed spaces (e.g. packages, containers, cavities) and form thereon an adsorption film which protects against corrosion.
Such VCI oils are described for example in the patents GB 919,778, GB 1,224,500, U.S. Pat. Nos. 3,398,095, 3,785,975 and JP 07145490A. Since these VCI oils emit volatile corrosion inhibitors and protect against corrosion via the gas phase even the regions of metal surfaces that are not covered with an oil, they differ considerably from preserving oils in which the corrosion protection properties are improved by the incorporation of non-volatile corrosion inhibitors which are thus effective only in direct contact. Such corrosion protection oils are described for example in the patents U.S. Pat. Nos. 5,681,506 and 7,014,694 B1.
It is known that all measures for the temporary corrosion protection of metals against the effect of neutral aqueous media or condensed water films have the aim of preserving the primary oxide layer (POL), which always exists on utility metals after first contact with the atmosphere, against chemical and mechanical degradation (cf. for example: U.S. Pat. No. 6,752,934 B2 and DE-OS 102007059 726 A1).
Since many amines already have a relatively high vapour pressure or sublimation pressure under normal conditions and are adsorbed in particular onto ferrous materials which are covered with a POL, they have already been put to early use as VCIs and said use is described in many patents. Mention is made primarily therein of the cyclic amines dicyclohexylamine and cyclohexyl-amine. However, in the patents GB 600,328, U.S. Pat. Nos. 2,419,327, 2,432,840, 4,051,066 and 4,275,835 cited by way of example, account is already taken of the fact that no reliable temporary corrosion protection can be obtained using amines alone, and therefore the use of amines is combined with further volatile active substances. One group of substances used for this includes oxidation agents which can act as passivators. Using such passivators, it is possible to achieve the situation whereby the POL is spontaneously recreated as an oxidic top layer on metal substances when it has been destroyed by partial chemical disintegration or local mechanical removal (abrasion, erosion) (cf. for example: E. Vuorinen et al., loc. cit. and U.S. Pat. No. 6,752,934 B2).
As such passivating oxidation agents, the nitrites as salts of nitrous acid have proven useful in practical corrosion protection. They have therefore also already been used for a long time as VCIs. In particular, the relatively readily volatile dicyclohexylammonium nitrite has already been used as a VCI for more than 60 years (cf. for example Vuorinen et al., loc. cit.) and is mentioned as a constituent of VCI compositions in numerous patents (for example: U.S. Pat. Nos. 2,419,327, 2,432,840, 2,534,201, 4,290,912, JP 62109987, JP 63210285 A and U.S. Pat. No. 6,752,934 B2).
However, its effect is more or less limited to the protection of ferrous materials, while the stability of the passive oxide layer of zinc and aluminium materials is often impaired.
With the aim of creating VCI packaging materials which can be used not only for ferrous metals but rather at least also for zinc-plated steels and aluminium materials, it has been proposed to combine nitrite/amine mixtures with further substances capable of sublimating, such as for example the salts of medium to weak, saturated or unsaturated carboxylic acids, cf. for example U.S. Pat. Nos. 2,419,327, 2,432,840. As a result, an improved protection of the customary Al and Zn materials is obtained when these are in contact with an aqueous medium or condensed water film, but at the same time the passivator properties of the nitrite are reduced by these species. It is known that the carboxylates in question build up pH buffer systems with a relatively high buffer capacity in aqueous media or condensed water films on metal surfaces with or without the simultaneous presence of an amine, depending on the respectively present carboxylic acid/salt system, and thus usually hinder the reducibility of oxidation agents. The passivation effect can then be achieved only when the concentration of the oxidation agent in question is set in comparative terms to be much higher than the amounts of the other active substances.
Since nowadays the practical use of said oxidation agents is regulated due to their more or less damaging effect on humans and the environment that has become known, and since there are occupational exposure limits (OELs) which must be adhered to with regard to the concentration in preparations (cf. for example classification of substances and preparations according to EC Directive 67/548/EEC including annual updates), VCI combinations containing excessive amounts of passivator can no longer be used.
Most of the VCI systems known to date, which contain simultaneously a nitrite and an amine, are also unable to provide the necessary reliability since they consume one another through chemical reactions. For instance, it has in the meantime been found that in particular the secondary amines and the compounds containing a cyclic nitrogen, such as for example morpholine and piperidine, which are introduced as VCI components are easily converted to N-nitroso compounds. These N-nitrosamines usually act as weak oxidation agents and promote the corrosion of the metals. Much more disadvantageous, however, is their carcinogenic effect, which prevents these VCI systems from being used on an industrial scale.
Specifically, when incorporating VCI combinations in mineral oils or synthetic oils, oxidation agents such as the nitrites are unsuitable in any case since they would cause a relatively quick oxidative decomposition of the base oil in question. Furthermore, the salts of the customary aliphatic and aromatic carboxylic acids which are known as VCIs are also not sufficiently soluble in oils. The formulations of VCI oils that have become known have therefore until now been limited mainly to the use of amines as VCI components (cf. for example: GB 919,778, GB 1,224,500, U.S. Pat. Nos. 3,398,095, 3,785,975 and JP 07145490 A). For instance, U.S. Pat. No. 3,398,095 claims mixtures which contain, besides sulphurised oleic acids, C6 to C12 alkylcarboxylic acids and C20 to C22 alkylsuccinic acids, additionally also dicyclohexylamine, morpholine, piperidine, hexylamine and/or phenyl-alpha-naphthylamine, while U.S. Pat. No. 3,785,975 highlights amine salts of diesters of ortho-phosphoric acid combined with alkenyl-substituted succinic acids, esters of unsaturated fatty acids, alkylcarboxylic acids, such as octanoic acid and morpholine as corrosion-inhibiting additives. Finally, JP 07145490 A claims preparations containing ethanolamine carboxylates, morpholine, cyclohexylamine and various sulphonates. However, since it is certain nowadays that said longer-chain carboxylic acids, like the esters of fatty acids and the sulphonates, do not evaporate from the customary mineral oils and synthetic oils at temperatures<80° C. under normal conditions, only the amines can be emitted from such preparations and become active as VCI components.
However, VCI oils from which only amines are emitted in the temperature range of interest of up to 80° C. are suitable only for the VCI corrosion protection of iron-based materials. In the case of zinc and aluminium, they are known to cause together with condensed water usually an excessive alkalisation of the surfaces, as a result of which considerable corrosion appears with the formation of zincates or aluminates, before finally the hydroxides and basic carbonates appear, which are usually known by the term “white rust”. By contrast, copper materials under the effect of amines frequently suffer from corrosion with the formation of Cu-amine complexes.